This invention relates to gas turbine engines and, more particularly, to a gas turbine engine having a variable fan nozzle for controlling a bypass airflow through a fan bypass passage of the gas turbine engine.
Gas turbine engines are widely known and used for power generation and vehicle (e.g., aircraft) propulsion. A typical gas turbine engine includes a compression section, a combustion section, and a turbine section that utilize a core airflow into the engine to propel the vehicle. The gas turbine engine is typically mounted within a housing, such as a nacelle. A bypass airflow flows through a passage between the housing and the engine and exits from the engine at an outlet.
Presently, conventional gas turbine engines are designed to operate within a desired performance envelope under certain predetermined flight conditions, such as cruise. Conventional engines tend to approach or exceed the boundaries of the desired performance envelope under flight conditions outside of cruise, such as take-off and landing, which may undesirably lead to less efficient engine operation. For example, the size of the fan and the ratio of the bypass airflow to the core airflow are designed to maintain a desired pressure ratio across the fan during cruise. However, during take-off and landing, the pressure ratio may change such that pressure pulsations occur across the fan (i.e., fan flutter). The pressure pulsations cause less efficient fan operation and increase mechanical stress on the fan, which ultimately causes an increase in fuel consumption and reduces the life expectancy of the fan.
Therefore, there is a need to control the bypass airflow over a wider variety of different flight conditions, such as take-off and lift-off, to enable enhanced control of engine operation.